1. Field of the Invention
The present invention concerns the processing of digital images and, more particularly, a method for filtering noise in a digital image sequence.
2. Description of the Related Art
Digital images are currently being used in numerous applications, including those related with such traditional acquisition devices as still and video cameras. It is to be expected that ever greater use of digital images will be made in such new generation devices as mobile multimedia communication terminals.
There exist numerous devices or applications that use digital images in sequence, that is to say, images acquired one after the other, separated by a brief interval of time and representing approximately the same real scene.
The speed with which the sequence is acquired, i.e., the number of images acquired in a given time interval, may vary according to the specific application; for example, this number is very large in digital video cameras (about 25 images per second) and smaller (about 15 images per second) in mobile communication terminals, which acquire the digital images and then transmit them in real time to a remote terminal.
It is well known that digital image acquisition devices, especially when they include CMOS sensors, will intrinsically introduce noise into the acquired images.
In digital image sequences noise not only degrades the quality of the images, but also reduces the encoding/compression efficiency. Indeed, the acquired image sequences have commonly to be encoded/compressed by means of encoding/compression techniques that operate in accordance with, for example, the MPEG standard or the H263 standard and nowadays are very widely used in the greater part of devices in the market today.
The encoding/compression efficiency becomes reduced by the presence of noise, because the introduced noise is typically in the form of random fluctuations that reduce redundancy both within an image and between images that are temporally close to each other.
There exist numerous filtering techniques intended to reduce or eliminate the noise present in an image sequence.
Numerous attempts have been made to develop efficient techniques for reducing the noise of a sequence by using various specific types of filters. Known digital filters include, for example, low-pass filters, median filters, adaptive spatial filters and recursive temporal filters with or without motion compensation.
Other prior art techniques seek to improve noise reduction efficiency in image sequences by having recourse to hybrid methods that combine digital spatial filtering with digital temporal filtering.
Though the known techniques for reducing noise in image sequences are satisfactory in many respects, they are also associated with numerous drawbacks and problems that are bound up with, for example, inadequate performance, processing complexity and excessive processing costs that make it difficult to employ them in portable acquisition devices of a commercial type.